The present invention relates, in general, to controlling the actuation of a downhole device and, in particular, to a hydraulic control assembly for actuating a hydraulically controllable downhole device using subsea intensification of a hydraulic fluid.
Without limiting the scope of the present invention, its background will be described with reference to subsurface safety valves as an example.
Subsurface safety valves are commonly used to shut in oil and gas wells in the event of a failure or hazardous condition at the well surface. Such safety valves are typically fitted into the production tubing and operate to block the flow of formation fluid upwardly therethrough. The subsurface safety valve provides automatic shutoff of production flow in response to a variety of out of range safety conditions that can be sensed or indicated at the surface. For example, the safety conditions include a fire on the platform, a high or low flow line temperature or pressure condition or operator override.
During production, the subsurface safety valve is typically held open by the application of hydraulic fluid pressure conducted to the subsurface safety valve through an auxiliary control conduit which extends along the tubing string within the annulus between the tubing and the well casing. For example, flapper type subsurface safety valves utilize a closure plate which is actuated by longitudinal movement of a hydraulically actuated, tubular or rod type piston. The flapper valve closure plate is maintained in the valve open position by an operator tube which is extended by the application of hydraulic pressure onto the piston. Typically, a pump at the surface pressurizes hydraulic fluid from a hydraulic fluid reservoir that is also at the surface. The high pressure hydraulic fluid is then delivered through the control conduit to a variable volume pressure chamber of the subsurface safety valve to act against the crown of the piston. When, for example, the production fluid pressure rises above or falls below a preset level, the hydraulic control pressure is relieved such that the piston and operator tube are retracted to the valve closed position by a return spring. The flapper plate is then rotated to the valve closed position by, for example, a torsion spring or tension member.
It has been found, however, that as oil and gas wells are being drilled in deeper water, the hydrostatic pressure of the column of hydraulic fluid in the control conduit approaches the closing pressure of typical subsurface safety valves. Accordingly, stronger springs are required to generate the necessary closing pressure such that a subsurface safety valve installed in a deep water well may be operated to the closed position. It has been found, however, that use of these stronger springs increases the opening pressure required to operate the subsurface safety valve from the closed position to the open position as well as the pressure required to hold the subsurface safety valve in the open position. This in turn requires that the entire hydraulic system used to control these deep water subsurface safety valves must be operated at a higher pressure.
Therefore, a need has arisen for an apparatus and method for actuating subsurface safety valves installed in deep water wells wherein the hydrostatic pressure of the column of hydraulic fluid in the control conduit does not approach the closing pressure of the subsurface safety valves. A need has also arisen for such an apparatus and method that does not require the use of stronger springs in the subsurface safety valve to generate high closing pressures. Further, a need has arisen for such an apparatus and method that does not require the use of hydraulic systems having higher operating pressures to generate the higher opening and holding pressures required to overcome the higher spring forces of stronger springs.
The present invention disclosed herein comprises a hydraulic control assembly and method for actuating a hydraulically controllable downhole device that is installed in a deep water well. Using the hydraulic control assembly of the present invention, the hydrostatic pressure of the column of hydraulic fluid in the control conduit does not approach, for example, the closing pressure of the subsurface safety valve. Accordingly, subsurface safety valves installed in deep water wells using the hydraulic control assembly of the present invention do not require stronger springs for closure and do not require higher hydraulic opening pressures.
The hydraulic control assembly of the present invention includes a hydraulic fluid source located on a surface installation that is used to supply low pressure hydraulic fluid. An umbilical assembly is coupled to the hydraulic fluid source. The umbilical assembly provides a supply fluid passageway for the low pressure hydraulic fluid. A subsea intensifier that is operably associated with a subsea wellhead is coupled to the umbilical assembly. The subsea intensifier receives the low pressure hydraulic fluid from the umbilical assembly and pressurizes the low pressure hydraulic fluid into a high pressure hydraulic fluid suitable for actuating the hydraulically controllable device. The subsea intensifier may have one of several power sources. A surface hydraulic power source may be coupled to the subsea intensifier via the umbilical assembly or a surface electric power source may be coupled to the subsea intensifier via the umbilical assembly.
In another embodiment of the present invention, the hydraulic control assembly of the present invention includes a subsea hydraulic fluid source. A subsea intensifier is operable to convert the low pressure hydraulic fluid from the subsea hydraulic fluid source into a high pressure hydraulic fluid suitable for actuating the hydraulically controllable downhole device. An umbilical assembly may be coupled between the surface installation and the subsea intensifier to provide electrical power to the subsea intensifier. Alternatively, a subsea battery may provide electrical power, in which case the subsea intensifier may be controlled via wireless telemetry.
The method of the present invention includes storing a hydraulic fluid in a reservoir located on a surface installation, supplying low pressure hydraulic fluid from the reservoir via an umbilical assembly to a subsea intensifier which is operably associated with a subsea wellhead and converting the low pressure hydraulic fluid into high pressure hydraulic fluid suitable to actuate the hydraulically controllable downhole device. Alternatively, the method includes storing the hydraulic fluid in a subsea reservoir, pressurizing the hydraulic fluid with a subsea intensifier and actuating the hydraulically controllable downhole device.